Since the discovery of negative refraction [1]} and transformation optics [2]}, and the rapid development of metamaterials [3]}, it has been widely appreciated that one wave is like any other. Although quantum mechanics has its peculiarities, there is nothing fundamentally different between the Dirac equation, Schrödinger equation, the classical Maxwell equations, or the equations of elasticity. The same topological arguments have therefore been applied to design of periodic electromagnetic materials supporting unidirectional interface states, dubbed `photonic topological insulators' [4]}, which has led to the fields of topological photonics [5]} and acoustics [6]}.
